Spread spectrum (SS) communications systems communicate information by encoding it with a pseudorandom (PR) sequence in such a manner that the frequency bandwidth of the encoded signal is spread. In this respect, spread spectrum communication is similar to wideband frequency modulation. A broadband frequency modulation communication system has the advantage that increasing the system bandwidth at constant transmitted power density increases the signal-to-noise ratio of the received signal. When encoded, the transmitted power of a spread spectrum signal is spread over a relatively large bandwidth and therefore the power spectral density is low, so that the signal power is low relative to the noise in a given bandwidth. Spread-spectrum communication differs from frequency modulation communication in that the SS signal power density is comparable to the ambient noise density. This attribute of SS communications makes it possible to "re-use" a portion of the electromagnetic frequency spectrum being used by other communication services. For example, a spread-spectrum communication system having a transmitted power comparable to the noise in a bandwidth may coexist with a television transmission system using the same bandwidth without affecting the television communication. This is termed an "overlay" mode of operation.
The spread spectrum receiver associated with the low power density SS transmission can receive the SS signal because it uses correlation with a locally generated pseudorandom sequence corresponding to the encoding sequence to "de-spread" the SS signal to a much smaller bandwidth, in which the signal-to-noise ratio (SNR) can be relatively high. In conjunction with the de-spreading of the signal bandwidth by means of the local pseudorandom sequence, the fixed frequency signals of the other communication services with which the spread spectrum system coexists in the overlay mode of operation (overlay service) are, in turn, de-spread to a wide bandwidth. This reduces the power density of the potentially interfering signals, so that only a small amount of power attributable to the potentially interfering signal exists in the narrow IF or baseband bandwidth of the spread spectrum receiver. The small amount of interfering power does not materially affect the received SS signal.
It is well known that spread spectrum transmissions can be overlayed on one another by the use of code division multiple access (CDMA). In the CDMA mode, each transmitter and receiver of a set of transmitters and receivers processing the same information uses a single pseudorandom sequence for encoding and decoding the transmissions. Other sets of transmitters and receivers processing other information encode and decode with different PR sequences.
Public safety officials such as fire and police officers often require lightweight, portable, low power "walkie-talkie" communication transceivers (transmitter-receiver combinations) for communication (transmission and reception) of analog information such as audio signals. Frequency modulation is frequently used for such walkie-talkie service. As mentioned, frequency modulation spreads the spectrum of the audio signal and may reduce noise by comparison with amplitude modulation communication. An advantage of frequency modulation over amplitude modulation is that the amplitude of the received and processed audio signal depends upon the frequency of received signal and not upon its amplitude. Consequently, the volume of the audio signal does not vary in dependence upon the locations of the transmitters and receivers, so long as a usable signal is received.
The Federal Communications Commission has authorized limited use of spread spectrum transmissions by public safety agencies and by the general public. One of the cheapest, simplest and lowest-power spread spectrum encoding schemes is direct sequence encoding in which the spreading waveform is a pseudorandom digital bit sequence. The pseudorandom sequence can be used to spread either the analog or audio signal itself or a digitized version of the analog signal. Using inexpensive pseudorandom chip generators capable of clock or chip rates of three or four Megachips per second (M chips/sec) to spread the analog audio signal, and assuming that an audio bandwidth of three KHz is adequate for most uses, an effective spread spectrum system having a spreading gain of greater than 1000 is possible. Spreading gain is the ratio of SS modulated bandwidth to information signal bandwidth. A spreading gain in the vicinity of 1000 provides substantial immunity from interference by the services sharing the common bandwidth in overlay service and reduces interference by the spread spectrum signal on the co-users. The overlay capability of a spread-spectrum communication system is particularly advantageous for public safety officials, who may be required to communicate with each other at the scene of a disaster which may be near any of a multitude of diverse interfering services. A particular disaster may be near a mobile radio base station producing substantial power, for example, at 49 MHz, while another may be near an antenna farm having several television and FM broadcast stations ranging in frequency from 54 to 800 MHz. However, a conventional direct sequence SS system in which a direct sequence spreads or modulates a baseband audio signal has a disadvantage by comparison with ordinary frequency modulation in that the volume of the received signal changes in response to the received power, as in amplitude modulation. This occurs because the magnitude of the received signal may change with distance and with changes in the positions of the transmitters and receivers relative to interfering objects such as buildings. The communication may also be adversely affected by rapid changes in received signal power (flutter) caused by rapidly moving vehicular traffic in the area.
Direct sequence spreading of a digitized (pulse code modulated) audio waveform is also possible. When an audio signal is digitized it is quantized both in time (sampling) and in amplitude. In theory, it is only necessary to sample the signal at a frequency which is twice the highest audio frequency component to be transmitted. However, in actual practice the standard sampling frequency is 8 KHz for a 3 KHz audio channel. Furthermore, an audio signal must be amplitude quantized to about 130 to 250 levels (7 or 8 bits) to provide a natural sounding communication. Thus, 7 or 8 bits must be transmitted during each sampling interval. The bit rate for audio transmission, then, must be (8 KHz).times.(8), which equals more than 64 Kbits/sec if bandwidth compression techniques are not used. Even using bandwidth compression techniques such as delta modulation, the least practical transmission rate must be in the range of 32 Kbits/second to achieve both low cost and natural speech sound. Considering that the bit rate of the PRS generator is in the range of 3 Mchips/sec, the spreading gain is EQU 3.times.10.sup.6 chips/sec/3.2.times.10.sup.4 bits/sec.apprxeq.100
Such a low spreading gain may result in interference by the spread spectrum system on the service using the bandwidth on which it is overlayed, and similarly the service may interfere with the spread spectrum system.
A simple communications system adapted for audio or other analog communication in an overlay mode is desired, which is capable of high spreading gain for reduced interference among co-users of the spectrum.